1) Field of the Invention
This invention relates to an internal combustion engine of the type that an intake air flow inducted through an intake port into a cylinder is formed into a swirl and an air-fuel mixture so swirling is then caused to burn.
2) Description of the Related Art
In general, a main body of a reciprocal internal combustion engine is primarily constructed of a cylinder head, a cylinder block and a crankcase. Received inside these elements are at least one cylinder with a piston fitted therein, an intake passage and exhaust passage communicable via an intake valve and an exhaust valve to a combustion chamber formed in an upper part of the cylinder, a valve system for driving the intake valve and the exhaust valve, a connecting rod for converting a reciprocal motion of the piston into a rotational motion and transmitting the latter to a crankshaft, etc.
In such an internal combustion engine, intake air inducted in a cylinder during an intake stroke is fed with fuel in an amount corresponding to the amount of the inducted air so that combustion energy is produced. This energy is received by a piston and, after converted to rotational energy, is outputted.
Depending on the manner of ignition for combustion, these internal combustion engines can be classified into diesel engines and the like, as compression-ignition internal combustion engines, and gasoline engines as spark-ignition internal engines. Depending on the engine strokes, on the other hand, they can also be classified into 2-cycle engines and 4-cycle engines.
Irrespective of the type of an internal combustion engine, the internal combustion engine is required to show fast operation responsibility and, while retaining good combustion stability, to be improved in combustion efficiency so that it can provide a sufficient output at low fuel consumption.
To improve the combustion efficiency of an internal combustion engine, it is effective to promote atomization of fuel and also mixing of the fuel and air in a combustion chamber. Proposals have hence been made as to the configuration of a top wall of a piston or a lower wall of a cylinder head, said top wall and lower wall forming a combustion chamber. It has also been proposed to design the manner of feeding of fuel in accordance with the configuration of the combustion chamber. Further proposals have also been made regarding the position of a spark plug.
For example, Japanese Utility Model Application Laid-Open (Kokai) No. HEI 3-52333 discloses an internal combustion engine of the cylinder injection type having an intake valve and exhaust valve in a cylinder head. With a view to achieving good scavenging effect even during low-load operation, good mixing of burnt gas and atomized fuel during high-load operation, and good combustion over a wide load range, the following construction is proposed there.
An intake opening of the cylinder head is provided with a mask for closing the intake opening on a side of the exhaust valve while the intake valve is opened. A top wall of a piston is provided with a raised portion on the side of the exhaust valve and also with a recessed portion on a side of the intake valve. A flow of inducted air is accordingly guided in such a way that the inducted air descends from the intake opening along an inner wall of the cylinder on the side of the intake valve, flows to the side of the exhaust valve along the top wall of the piston and then ascends along the inner wall of the cylinder on the side of the exhaust valve, so that a so-called vertical vortex is formed. A fresh air flow is therefore ensured to have a sufficient flow velocity in an attempt to provide sufficient scavenging effect even during low-load operation.
In addition, a squish is produced at the raised portion on the top wall of the piston so that the mixing of burnt gas and a spray of fuel is promoted to achieve good combustion even during high-load operation.
Further, Japanese Patent Application Laid-Open (Kokai) No. He 4-58030 discloses a spark-ignition internal combustion engine of the cylinder injection type. It is proposed to form a recessed portion on a side of intake valves in a top wall of a piston and to ignite fuel, which has been injected into the recessed portion, by ignition means. Another proposal is made there with respect to the timing of the above ignition.
Japanese Patent Application Laid-Open (Kokai) No. HEI 4-6535, on the other hand, discloses a spark-ignition internal combustion engine of the cylinder injection type. It is proposed to form a top wall of a piston with a deep dish portion, which corresponds to the recessed portion described above, and a shallow dish portion located around the deep dish portion and further to form a bottom of the shallow dish portion into an upwardly convex surface. Fuel, which has been injected toward the shallow dish portion, is caused to widely spread in to a thin layer along the upwardly convex surface. As a result, evaporation of the fuel is promoted, so that production of smoke is reduced and a uniform air-fuel mixture is formed over a wide range to reduce knocking.
If the area of the shallow dish portion is enlarged with a view toward improving combustion during high-load operation, in other words, the velocity of evaporation of fuel, the injected fuel enters the shallow dish portion and spreads too much in the shallow dish portion during medium-load operation. This makes it difficult to allow a flame to propagate from the deep shallow portion, leading to the problem that combustion is impaired and HC is produced. To overcome this problem, the following construction is proposed in U.S. Pat. No. 5,109,816.
An outer edge portion is formed between the deep dish portion and the shallow dish portion located around the deep dish portion, with an edge portion interposed between the outer edge portion and the deep dish portion. During medium-load operation, fuel is injected in such a way that the fuel so injected hits the edge portion between a fuel-evaporating groove in the deep dish portion and the outer edge portion. After hitting the edge portion, the fuel is divided into two parts, one being on a side of the fuel-evaporating groove and on a side of the outer edge portion, as thin liquid layers. The evaporation of the fuel is therefore promoted, thereby preventing production of smoke which may occur due to localized over-enrichment of fuel. The liquid layer formed inside the fuel-evaporating groove is caused to evaporate by a swirl, whereby an air-fuel mixture to be ignited is formed near a spark plug. Evaporation of the liquid layer at the outer edge portion is promoted because the liquid layer is thin. Further, the liquid layer at the outer edge portion is prevented from over-flowing to the side of the shallow dish portion by a side wall of the outer edge portion. As a result, an air-fuel mixture of an appropriate fuel concentration is formed inside the outer edge portion, thereby ensuring propagation of a flame from the deep dish portion to the outer edge portion and further to the shallow dish portion.
To cause an air-fuel mixture to center around a spark plug in a cylinder-injection, internal combustion engine making use of an air blast valve, the following construction is proposed, for example, in U.S. Pat. No. 5,115,774.
A groove is formed in a top wall of a piston. During a latter part of a compression stroke, fuel is injected together with compressed air from the air blast valve toward the groove. The width of the groove as measured in a direction perpendicular to the direction of injection of the fuel is narrower than the longitudinal width of the groove and both side walls defining the transverse width of the groove are located slightly outside a range of the fuel so injected. The injected fuel is therefore prevented from spreading sidewards, whereby the air-fuel mixture is allowed to center around the spark plug.
To prevent injected fuel from directly hitting an inner wall of a cylinder and hence to avoid seizure of a piston in a cylinder-injection internal combustion engine, the following construction is proposed, for example, in U.S. Pat. No. 5,115,776.
A groove is formed in a top wall of a piston and fuel is injected toward the groove. Described specifically, two fuel injection valves are provided, one directing exactly toward the groove when the position of the piston is high and the other directly precisely toward the groove when the position of the piston is low, whereby the fuel can be injected toward a center of the groove irrespective of the position of the piston.
To form a combustible air-fuel mixture around a spark plug with a small injection amount of the fuel in a cylinder-injection, internal combustion engine, the following construction is proposed, for example, in U.S. Pat. No. 5,127,379.
In a top wall of a piston, a groove is formed extending from a point below a spark plug to another point below a fuel injection valve. In spark-plug-side parts of walls of the groove, arcuate stepped portions are formed, respectively. The fuel, which has been injected toward the walls of the groove from the fuel injection valve, is therefore guided by the walls of the grooves so that the fuel is allowed to center around the spark plug. At this time, the fuel stuck on the walls of the groove is allowed to evaporate and center around the spark plug while being separated from the walls of the groove.
In each of the conventional examples described above, a dimensional reduction of a combustion chamber has been achieved by forming a recessed portion only in a part of a top wall of a piston. Such a combustion chamber will hereinafter be called an "eccentric combustion chamber". This compact combustion chamber is a means for improved combustion. In this sense, each conventional example can be said to be a representative engines in which a certain improvement has been made in combustion. If a further improvement in combustion is desired, effective formation of a swirl inside a combustion chamber is indispensable. Upon formation of such a swirl inside a combustion chamber, it is necessary to suitably design not only the configuration of a top wall of a piston but also the configuration of an intake port through which intake air is fed to the combustion chamber (and, if necessary, the configuration of an intake passage communicating to an upstream end of the intake port). It is particularly preferred to employ such a construction that the top wall of the piston and the intake port can cooperated with each other to enhance a swirl. Regarding an eccentric combustion chamber capable of achieving a compact combustion chamber, the top wall of its piston is, as disclosed in Japanese Utility Model Application Laid-Open (Kokai) No. HEI 3-52333, is formed in a configuration suited for the formation of a vertical swirl in a downward direction, that is, toward the top wall of the piston on an intake side inside the combustion chamber.
According to the structure of the intake port disclosed in each of the conventional examples described above, it receives at the upstream side thereof intake air from the intake-side side wall of the cylinder head and is suddenly bent downwardly in the cylinder head to introduce the intake air into the combustion chamber. Such an intake port structure cannot provide the intake air flow, which has been introduced into the combustion chamber, with any sufficient downward flow component, thereby making it difficult to sufficiently form the above-described vertical swirl.
Paying particular attention to an internal combustion engine of the cylinder injection type, an injector is mounted on a cylinder head in such a way that the injector is oriented directly toward a combustion chamber. Taking into consideration the requirement that a main body of the injector and the fuel fed through the injector should be kept apart from heat, it is desired to arrange the injector at a position as remote as possible from the side of an exhaust port which becomes hot. With the construction that a passage extending from an intake port opens through a side wall of a cylinder head, however, it is often difficult to adopt the above-described arrangement.
In the case of an internal combustion engine of the spark ignition type, it may be contemplated to achieve so-called lean burn and hence to improve the combustion efficiency by causing a rich air-fuel mixture to partly gather a spark plug. In this case, it is necessary to arrange an injector so that fuel can be injected toward the spark plug. With the construction that a passage extending from an intake port opens through a side wall of a cylinder head, however, it is often difficult to adopt the above-described arrangement.
Multi-valve engines, led by 4-valve engines in which each cylinder is provided with two intake valves and two exhaust valves, have become popular especially in recent years. In such a multi-valve engine, the spaces around its intake and exhaust passages become narrower so that a still more stringent limitation is imposed on the space for the arrangement of the individual parts described above. This makes it more difficult to arrange the individual parts at optimal positions, respectively.
To efficiently perform combustion with a small amount of fuel under lead burn conditions, it is more effective to form a compact combustion chamber and to make a rich air-fuel mixture gather in part near a spark plug so that combustion is achieved.
When a reduction in fuel consumption is achieved especially by lean burn, it is impossible to substantially reduce NOx (nitrogen oxides) contained in exhaust gas despite the use of a lean air-fuel mixture. It is hence contemplated to add an exhaust gas recirculation system (hereinafter called the "EGR system") in which a part of exhaust gas is caused to flow into an intake pipe and is burned again to reduce the amount of NOx. In particular, a cylinder-injection, internal combustion engine is generally suited for an EGR system so that the amount of NOx can be reduced by a large volume of EGR gas.
In a lean-burn, internal combustion engine, it is general that the intake pressure is high and the difference between the intake pressure and the exhaust pressure is small. It is therefore difficult to feed sufficient EGR gas to each intake port, resulting in the problem that the EGR gas cannot be provided with any sufficient flow rate.
Especially, a long EGR passage leads to a lower EGR gas temperature and also to smaller EGR gas flow rate due to flow resistance.